If it does sound like a bad muffler, I would check the condition of the flex pipe. Its hard, if not impossible, to pin point noises over the internet, but based on your description, it would have to be load conditioned, so it would need to be a flexible part issue. The flex pipe and intake tube are the only two things in that area that will be "loud" You might have have an issue with a vacuum on the variable geometry intake. But that would't be as loud.

I can't see how a mechanic could miss this if it is a exhaust, but the flex pipe can reseal it self.